The present invention relates to a digital amplifier (class D amplifier), and particularly relates to an overcurrent detection circuit of the digital amplifier.
In recent years, digital amplifiers each provided for converting an analog signal such as a music signal into a pulse signal, amplifying the power of the pulse signal and outputting the amplified pulse signal through a low pass filter have been used broadly due to their high efficiency.
As a protection circuit in such a digital amplifier, there is known a current limiter (Non-Patent Document 1).
FIG. 7 is a view showing a configuration example of such a digital amplifier provided with a current limiter.
In FIG. 7, the reference numeral 51 designates an input amplifier; 52, a PWM modulation portion for generating a pulse signal in accordance with the amplitude of an input analog signal and outputting a driving signal for driving a class D amplifier stage; 53, the class D amplifier stage (switching stage) constituted by switching devices QH and QL which are power MOSFETs or the like; C51 and C52, decoupling capacitors; 54, an output filter (low pass filter) for reproducing an analog signal from a pulse signal outputted from the class D amplifier stage 53; 55, a speaker; and 56, a protection circuit for performing protection operation when the protection circuit detects the fact that a drain current Id flowing into the power MOSFET QH reaches a predetermined threshold value (limit value) or higher.
A current detection resistor Rd is inserted between a drain of the high-side MOSFET QH of the class D amplifier stage 53 and a positive power supply +V, so that a voltage Vd(=Rd·Id) is generated between the opposite ends of the current detection resistor Rd due to the drain current Id flowing into the MOSFET QH. Connection is made such that a voltage obtained by dividing the voltage Vd by resistors R51 and R52 is applied between a base and an emitter of a transistor Q51 of the protection circuit 56. As soon as an excessive drain current Id exceeding the limit value flows into the MOSFET QH due to a short-circuited load or the like, the transistor Q51 is electrically connected to perform protection operation for protecting the switching device QH.
Incidentally, although the current detection resistor Rd is inserted in the aforementioned description, there has been also a proposal in which protection operation is performed instead in accordance with a voltage generated between a drain and a source of an MOSFET and detected directly when the MOSFET is on (Patent Document 1).
Although an overcurrent flowing into the high-side switching device QH is detected in the example of FIG. 7, an overcurrent flowing into the low-side switching device QL may be detected instead or both the overcurrent flowing into the high-side switching device and the overcurrent flowing into the low-side switching device may be detected.
On the other hand, an analog power amplifier uses a PC limiter whose current threshold value of the current limiter has dependence on an output voltage.
FIG. 8 is a view showing a configuration example of such an analog power amplifier provided with a PC limiter.
In FIG. 8, resistors R62 and R63 are connected in parallel with an emitter resistor R61 of an output transistor Q61. This connection point between the emitter resistor R61 and the resistors R62 and R63 is connected to a base of a transistor Q63. The base of the transistor Q63 is connected to the ground through a resistor R64 and a diode D61.
In the circuit configured thus, a voltage is generated between the opposite ends of the resistor R61 in accordance with a load current Ic of the output transistor Q61. A partial voltage obtained by dividing this voltage by the resistors R62 and R63 and a partial voltage obtained by dividing the output voltage by the resistors R63 and R64 (voltage reverse to the partial voltage obtained by the resistors R62 and R63) are generated between the opposite ends of the resistor R63. When a voltage obtained thus between the opposite ends of the resistor R63 is higher than a voltage with which the transistor Q63 can be electrically connected, the transistor Q63 is electrically connected so that a transistor Q64 is electrically connected. Thus, protection operation is started.
In this manner, the load current Ic of the output transistor Q61 is detected from the voltage generated between the opposite ends of the emitter resistor R61 of a power amplifier output stage and the amplitude of the output voltage is acquired by the resistor R63. Thus, protection operation is performed in response to a large current value when the amplitude of the output voltage is large. On the other hand, protection operation is performed in response to a small current value when the amplitude of the output voltage is small.
According to the PC limiter, for example, when the load is short-circuited, the current limit value can be set to be so small that a load imposed on an output device can be reduced. Consequently, the PC limiter is more effective than the aforementioned current limiter in terms of protection of the output device.    Patent Document 1: Japanese Patent Laid-Open No. 2003-060449    Non-Patent Document 1; Tamotsu Inaba, “Basics and Practices of Power MOSFET Applications”, CQ Publishing Co. Ltd., Nov. 1, 2004, pp. 68-72
In order to design a digital amplifier to take a large maximum output, a threshold value of a current to be detected by a current limiter has to be set in accordance with a maximum load current of an MOSFET in an output stage. As a result, the MOSFET in the output stage may be broken because protection operation is not carried out, for example, when a speaker is damaged in a short mode during reproduction with a small signal, when the speaker with a low AC impedance beyond the real capability of the amplifier is sounded with large amplitude, or when the speaker is used for a long term with a part of speaker wires being left in contact with a negative terminal, a chassis, etc. due to customer's faulty wiring. It is general that a customer may arrange faulty wiring or there may be a frequency band in which the AC impedance of the speaker is lower than a nominal value. In such a case, it is necessary to protect the set from damage.
In the digital amplifier, the waveform of a power supply voltage switched on and off repeatedly appears simply short of a coil of an output filter. Any output amplitude of a sine wave etc. is expressed by the duty ratio of the switching. Accordingly, there is a problem that the aforementioned PC limiter serving in the analog amplifier cannot detect the output amplitude of a final signal.
Further, the background-art current limiter in the aforementioned digital amplifier performs protection operation simply by switching on and off an output transistor. It is therefore impossible to perform an operation to monitor a load current in any time so as to limit the maximum output amplitude in consideration of the balance between the load current and the real capability of the amplifier.